The term "rotary aircraft" is used to designate conventional rotary-wing aircraft such as helicopters having fuselages designed for independent non-rotational positioning with respect to a rotary wing and to aircraft lacking such fuselages.
It is well know to aeronautical engineers familiar with rotor design that a fixed-pitch rotor is inherently unstable in hover. A rotor comprising a hub with a plurality of blades, which are fixed in pitch with respect to the hub, will, when operated in hover (i.e. without horizontal movement), be disturbed by any incident wind perpendicular to the axis of rotation. The disturbance is characterized by the aerodynamic center of the rotor moving from the axis of rotation towards the source of the wind. The resulting aerodynamic imbalance causes the rotor to pitch. Because it is spinning, gyroscopic forces also cause the rotor to roll in a direction dependent upon the sense of rotor rotation. These combined forces act to cause the rotor to follow a circular flight path and this can cause the rotor to increase in pitch and roll in an uncontrolled manner.
In the past, means for stabilizing the spinning rotor of a helicopter or other rotary aircraft in hover have involved varying the pitch of individual rotor blades cyclically by a system controlled actively by a mechanical or electronic gyroscopic sensor. In hover, the blade pitch is varied cyclically so as to reduce lift at the rotor's upwind side to a value equal to that on the lee side and thus keep the resulting aerodynamic center of the rotor at the axis of rotation.
It would be desirable to provide a simple, low-cost means for stabilizing a fixed pitch rotor in hover.
It would further be desirable to provide a means for passively stabilizing a fixed pitch rotor in hover.
It further would be desirable to provide simple, inexpensive, rotary aircraft of minimal height, size, complexity and expense capitalizing upon such passive stability.